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Specifying encoders for hazardous locations

Jan. 1, 2012
Highly explosive applications sometimes call for speed or position feedback. When they do, specially designed encoders must be properly integrated to prevent catastrophe.

One of the most challenging tasks design engineers can face is designing measurement or control systems to operate safely in hazardous environments. These environments may contain flammable or explosive gas, dust, or fiber — and involve oil and gas extraction, mining, spray painting, or chemical, grain, or textile processing.

Specially designed and rated equipment is required in these locations; more specifically, encoders are often needed for speed or position feedback, and must be designed and operated within the correct hazardous area rating.

To ensure the safety of systems operating in hazardous locations, governments worldwide have established regulatory bodies that have developed testing and certification standards and product marking requirements. Some countries have coordinated the certifying process, creating reciprocity agreements with the different agencies, and allowing certifying bodies of one country to test and certify to standards of another country.

Even so, with various classifications of hazardous locations, different nations have developed standards for equipment installed in these environments — and a patchwork of requirements has evolved.

So, as an engineer needing to specify an encoder amid the maze of regulations, where should one begin? Start by understanding how a hazardous location is defined, and then determine what rating is required for the installation at hand.

Classifying a hazardous location

An area can be classified as hazardous based on the following factors: First, is there a possible presence of an explosive or flammable agent? If so, what is that agent? Second, is the explosive atmosphere present under normal operating conditions, or only if there is a malfunction or unusual occurrence that might release an explosive agent into the area? Third, in what part of the world will the equipment be installed?

In fact, the United States (ANSI/NFPA), Canada (CEC), Europe (CENELEC), and much of the rest of the world (IEC) use this approach to area classification.

Once engineers have answered the above questions, they can determine both the hazardous area rating needed and the certifying body that must approve the equipment or installation.

Two main classification systems exist: The Class, Division, and Group system is used to classify hazardous locations in the U.S., Europe, and other parts of the world use the Class and Zone system, also called the CENELEC system. Let's explore both of these classification systems in more detail.

Types of hazardous agents

Once the possible presence of an explosive agent is determined, the engineer must classify that material into one of three categories — to provide the top level rating necessary for the encoder to be integrated. Class I includes flammable gasses, liquids, and vapors; Class II covers combustible dusts; Class III encompasses ignitable fibers and flyings.

Materials within these classes have different levels of ignitability, so within each Class/Division or Class/Zone there are Groups and Temperature Codes that correspond to the ignition temperatures of the flammable agents.

With only minor exceptions, Zone 1 in the CENELEC classification system is the same as Division 1 in the U.S., and Zone 2 CENELEC is virtually the same as Division 2 in the U.S.

Determining exposure levels

The conditions under which exposure to the hazardous agent is expected also affect encoder rating requirements. If the explosive agent will be present continuously or intermittently under normal operating conditions, it is defined as a Division 1 location in the U.S. and a Zone 0 or Zone 1 location in other parts of the world. Equipment in Division 1/Zone 0 and 1 environments must pass stringent testing and cannot create an ignition hazard, even in the event of equipment failure.

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Installing and maintaining equipment in these saturating environments is quite stringent, and can be costly and complicated. If the ignitable agent is likely to exist under abnormal operating conditions where a malfunction has occurred, it is classified as Division 2 in North America and Zone 2 in European applications. This rating has less stringent testing requirements, and installation and maintenance tend to be less costly.

Where will this equipment get installed?

Many countries around the world operate under different hazardous location standards and classification guidelines. Certain regions in the world have coordinated, or harmonized, their standards, but there is no single worldwide hazardous location standard. In Europe, CENELEC is the body that coordinates the standards of member nations, and ATEX is the directive from CENELEC that governs hazardous locations. In the U.S., the National Fire Protection Agency (NFPA) and ANSI produces the National Electric Code (NEC) that codifies hazardous location installations. Other countries are members of the International Electrotechnical Commission (IEC), which also sets hazardous location standards. CENELEC/ATEX and IEC standards are largely the same, although products must be certified and registered for each if they are expected to operate within both jurisdictions.

Determining a design's final installation location may seem like a simple step. However, many end users will need an encoder installed to suit their local hazardous area: If a builder is an OEM that ships machinery worldwide, it can be difficult to pinpoint the equipment's final destination. Often, this means the engineer will need multiple agency ratings on the encoder being installed. By having multiple ratings on one product, this allows much greater flexibility as to where the product can be used.

Operating in explosive environments

A variety of protection methods are available to suit explosive settings. The most recognized methods and their CENELEC designations are:

  • d — Flameproof (similar to explosion proof
  • p — Pressurization
  • q — Powder filling
  • o — Oil-immersion
  • e — Increased safety
  • i — Intrinsic safety
  • n — Nonincendive (or non-sparking)
  • m — Encapsulation

For encoders, the most common methods are explosion-proof construction, flameproof, nonincendive, and intrinsic safety.

  • Explosion-proof construction.

    With explosion-proof equipment, the encoder is contained in an enclosure that can withstand an internal ignition of the most volatile gas-to-air mixture that can penetrate into the enclosure. The enclosure must contain the explosion without sustaining damage. Further, the housing is evaluated to ensure that heated gasses from the explosion that pass through any gaps or joints in the assembly are cooled sufficiently — so they won't ignite explosive gasses outside the housing in the event of an explosion.

    Explosion-proof installations must be connected using a gas-tight conduit or special explosion-proof cable for electrical signals. Explosion-proof encoders can be rated for Division 1 or Zone 1 installations, where the explosive agent is present during normal operation.

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  • Flameproof construction.

    Flameproof d is a protection method defined by CENELEC/ATEX and IEC. It is similar to the explosion-proof North American protection method, in that it uses similar construction principles to ensure the enclosure can withstand an internal ignition of volatile gas-to-air mixtures. Flameproof encoders can be terminated with a certified seal and cable output. (Explore the differences between explosion proof and flameproof encoders in the sidebar.)

  • Intrinsic safety.

    The intrinsic safety protection technique uses an intrinsic safety barrier (also called an associated apparatus) that limits the maximum energy that can be delivered into the hazardous location. Intrinsically safe devices also have limitations on the amount of energy that can be stored within their bodies.

    Intrinsically safe equipment ensures that even in a double fault condition, there will not be enough energy supplied to the encoder or stored in the encoder to create a spark or heated surface to ignite the gas, vapor, dust, or fiber in the hazardous area.

    When used in compliance with spacing guidelines and installation requirements noted in the NEC, intrinsically safe installations can be made with standard signal cables. Note that encoders that use this method must use intrinsic safety barriers in their installation: An intrinsically safe encoder installed without an intrinsic safety barrier is not an intrinsically safe system.

  • Nonincendive.

    The nonincendive protection technique is acceptable in Division 2 and Zone 2 installations where explosive agents are present only in abnormal operating conditions. Nonincendive apparatus is evaluated to assure that no sparking or hot surfaces will occur in the device under normal operating conditions (unlike an intrinsically safe encoder, which is evaluated under fault conditions). Nonincendive devices must be installed using Class 2 wiring methods in the U.S. per the National Electric Code with similar requirements in other jurisdictions.

  • Refer to the illustration on the previous page of an offshore oilrig: Location one represents a Division 1/Zone 1 environment, because oil or explosive gasses can be released at the drill floor. Therefore, these hazardous materials are expected to be present under normal operating conditions, resulting in a Division 1/Zone 1 classification.

    Location two is the area in which gas from the Division 1/Zone 1 area will dissipate. This allows the area to be classified as Division 2/Zone 2, where hazardous agents are not likely to exist under normal operating conditions. Note that the example photo shows this location as having a wind wall, which functions to protect exposed equipment from inclement weather. This piece of equipment doesn't necessarily change the area rating, but does affect how far this rating is required to extend. Why? If a wind wall is present, it can contain potentially volatile gases, not allowing them to completely dissipate. Therefore, the entire interior of the wind wall is considered Division 2/Zone 2, as well as 1.5 meters above it. If the wind wall was not present, the Division 2/Zone 2 area would extend only up to 3 meters from the bell nipple on the drill floor.

    Locations three and four both represent tanks or pits that catch any mud or other material extracted as a result of drilling. This material could potentially release gasses that have been dissolved within it. As a result, these tanks could accumulate explosive, heavier-than-air gasses, requiring these areas to be classified as Division 1/Zone 1 areas.

    This type of hazardous location is not simply limited to major industrial applications. For example, a mechanic's garage pit could be considered a Division 1/Zone 1 environment. Accumulation of heavier-than-air gasoline fumes can accumulate in the pit under normal operating conditions — thus requiring top-level hazardous area certification.

    For engineering assistance with encoder applications in hazardous environments, call (805) 968-0782 or

    An important difference: Explosion-proof versus flameproof encoders

    The terms “explosion proof” and “flameproof” are often used interchangeably — though there are distinct differences between the two. The term explosion proof is used for North American-based approvals such as ANSI and NFPA. In contrast, flameproof is a term used for IEC and CENELEC/ATEX approvals.

    The major physical difference between explosion-proof and flameproof construction is in the termination. Explosion proof necessitates the equipment be terminated using a certified conduit fitting. Flameproof allows for termination using a certified cable gland and cable. Therefore, encoders with explosion-proof construction can be rated internationally by multiple agencies in Division 1, Zone 0 or Zone 1 environments. Flameproof encoders with cable outputs and gland seals, however, are not certified for use in Division 1 hazardous areas in North America.

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